This invention relates to holographic film for recording multicolor volume holograms, especially full color volume reflection holograms, and to manufacturing techniques for such film, and to corresponding methods of recording multicolor and full-color volume, in particular volume reflection holograms.
Volume reflection holograms utilize the principle of Bragg diffraction. An interference pattern created in the zone of intersection between a diffuse object beam and a coherent reference beam from the same laser, in the form of a standing wave, is recorded in a high-resolution “volume” recording material. Planar interference “fringes” are thus created in the form of refractive index modulation within the depth of the recording material.
This resembles the method used by Lippmann in the early part of the last century to create photographs with color effects at a time when conventional photography was restricted to “black and white” reconstruction of a scene.
This ‘volume reflection hologram’ phenomenon occurs when the orientation of the recording material, with respect to the position of the interference pattern, means that the alternate planes of high and low intensity caused by constructive and destructive interference, intersect the recording material in such a way that they are predominantly parallel with its surface in one plane.
The planar fringe structure, in the form of refractive index modulation, resembling alternate pages of a book, forms a dielectric wavelength selective reflector. Individual fringe planes in the form of alternate high and low index layers in the volume of the emulsion are spaced at half-wavelength intervals throughout the depth of the recording layer.
Light of a wavelength corresponding to twice the fringe spacing of the grating will be reflected, since its waves reflected from a first layer will interfere constructively with light from the next layer. Light of other wavelengths will interfere destructively and thus no sum reflection occurs.
In a transmission hologram, these fringes are predominantly perpendicular to the surface with the resulting creation of a linear fringe structure which has given rise to the concept of surface relief holograms which form the basis of the embossed hologram industry. In this case diffraction is in the form of a dispersive action which means light is diffracted at an angle in accordance with its wavelength, and thus these holograms have a quite different appearance to those under discussion in this document, since they have the familiar “rainbow” color appearance.
In the case of silver halide recording material, the fringe recording comprises layers of high refractive index component in the form principally of redistributed silver bromide whose pure crystalline form has an index of 2.23, interspersed with lower index planes of gelatin with index as low as approximately 1.5.
The degree of “index modulation” Δη, is responsible for controlling the efficiency of the reflectivity of these individual layers, i.e. the higher the reflectivity of each, the lower the number of planes necessary to achieve the desired diffraction efficiency of the hologram.
In the case of photopolymer recording material, the necessary index modulation is achieved by the mobility of the molecules of monomer compound which polymerize into a high index areas in microscopic zones of the fringe structure at positions where constructive inference between the hologram object and reference beams results in high intensity radiation; thus catalyzing polymerization of the monomer. Other functional systems are feasible in terms of chemical technology but the common factor is that light exposure is in some way leading to index modulation of the layer.
Manufacturers such as Pilkington (St. Asaph, N. Wales) have utilized thick emulsion to produce holographic optical elements for the purpose of “head-up displays” for fighter aircraft to enable a pilot to have a simultaneous view of his instrument panel with an otherwise unattenuated view through the screen. Only light of the very specific single wavelength of the panel illumination is reflected by the hologram in this case, so that almost all the natural light coming from outside the screen is attenuated from his view of the scene outside the cockpit, unlike the action of say a semi-silvered mirror element, which would reflect the panel light equally well, but would seriously reduce the brightness of the view of the exterior with obvious consequences.
In an image hologram for decorative, archival, or security purposes, however there is less desire for monochromaticity, especially since there may be a general desire to reflect light of a larger bandwidth, especially in the case of holograms for the former and latter purposes, in order to utilize as much of the available ambient light as possible to assist the casual viewer. As a result, the manufacturers of commercial silver halide materials, such as Agfa Gevaert with their Holotest materials, have generally coated their emulsion at a thickness of 7-8μ.
Workers such as Hans Bjelkhagen (Centre for Modern Optics, De Montfort University) have used a mixed (white) laser beam to illuminate an object in the “Denisyuk” mode of reflection holography wherein a spread and filtered white laser beam is incident upon a high resolution silver halide recording plate such as those provided by Slavich. Positioned close behind the recording plate is an object which is typically a valuable artifact such as a ceramic urn or vase etc. Laser light passing directly through the plate, including that scattered forward by the slightly opalescent emulsion, is incident upon the object and is then reflected as a diffuse wavefront predominantly back towards the recording emulsion such that it interferes with the incoming ‘reference’ beam to create a standing wave of interference, provided that the object and the whole optical system is absolutely stationary.
The standing wave of interference is basically a complex structure of alternate planes of high and low intensity illumination, which result from constructive and destructive interference between the object and reference beams travelling in predominantly opposite directions.
The recording plate is able to make an image of this interference pattern throughout its surface and depth. After development, the resulting fringe pattern will resemble the pages of a book with alternate high and low density of silver, nominally with a sinusoidal transverse profile. A recording of phase information, unlike the amplitude recording method of a conventional photograph, does not normally bear a visual relationship to the subject matter, whose appearance is only reconstructed when a suitable beam of light is incident upon the plate after processing is complete.
Typically the emulsion is then treated with a bleach solution which is able to change the black silver amplitude recording into a translucent phase structure wherein the standing wave originally recorded is represented by a refractive index modulation such that little of the incident light is absorbed or attenuated, but much of it is refracted by the grating.
Bjelkhagen has carried out a great deal of research published in his book “Silver Halide Emulsions for Holography and their Processing” in an attempt to find chemical means to provide improvements to the diffraction efficiency of the holograms made in this process.
The concept of diffusion transfer refers to the bleaching mechanism where silver ions are created by oxidation of developed silver metal and then encouraged by their environment to combine with a bromide ion and deposit themselves upon a local undeveloped emulsion crystal, thus transferring material from one zone to leave a density void whilst increasing the mean refractive index of the neighboring zone.
To date, recording material producers such as Fuji Hunt, Slavich and Color Holographic have taken the approach of sensitizing their emulsion with a range of spectral sensitizers so as to create a homogeneous photosensitive layer capable of reacting to exposure by lasers of wavelengths suitable to create recordings. These have been referred to as “panchromatic” films but in photographic terminology this term is usually used to describe materials which allow the balanced recording of all of the natural colors in a scene.
In a color holography system intended to record light from three specific monochromatic laser sources, spectral sensitizers can be incorporated with narrow band sensitivity peaks designed to coincide with the lasers without the need to design an overlap to enable sensitivity to all colors. So we could actually term the holographic material as “polychromatic” rather than “panchromatic”. A multilayer material for a color hologram is described in EP0562839.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for addressing problems relating to full color holographic image reproduction.